Marine or ship propulsion has been achieved in a variety of ways over time, including the use of propulsor elements such as propellers or waterjet impellers. Some of the primary challenges in designing a ship propulsion system include the matching of the propulsor elements (propellers or waterjet impellers) to the characteristics of the hull form, mission requirements, and the characteristics and limitations of the prime movers (e.g. diesel engines, gas turbines, electric motors). This is further complicated by the need to “balance” the performance of the system over the operating range of the prime mover.
Historically, ship propulsion systems have been optimized to address a key performance point for the application. For example, Ship Assist Tugs are normally optimized to maximize stationary pulling power, referred to as Bollard Pull, but in reality spend relatively little of their duty cycle at this operating point.
Sports fishing boats and Military Patrol Boats on the other hand are normally optimized for top-end speed. Accordingly, the most efficient and affordable installations have prime movers, reduction gears, and fixed pitch propellers or waterjets that are selected to maximize this desired performance characteristic, and most often sacrificing better performance at “off-design” operating point where they spend most of their time.
Examples of prior art that attempt to address this conundrum include the implementation of Controllable/Reversible Pitch Propellers and the implementation of electric drive systems.
The former is a common attempt at solving this problem but the trade-offs are: higher system acquisition cost; propeller blade shape that is optimized for top speed or Bollard Pull characteristics but is less efficient when operating outside this range; and larger propeller propulsor hub size with corresponding reduced overall efficiency.
The use of electric drives has the trade-off of being significantly higher in acquisition cost and has a lower operating efficiency over the entire operating range as a result of the mechanical-to-electrical power conversion.
An additional and more substantial challenge has been identified in Naval Ship applications. Worldwide, these Naval Ships have evolved into faster, smaller, more agile vessels, capable of operating at higher speeds in shallower coastal environments. Examples include the US Navy's Littoral Combat Ship and Joint High Speed Vessel. These are smaller, high horsepower ships capable of achieving speeds in excess of 35-40 knots. Prior art in the form of conventional single-impeller waterjets has been significantly challenged to “get the horsepower into the water” without causing destructive cavitation and without exceeding the space available on the transom of a narrow, high speed hullform.
Prior attempts to address this challenge using planetary gears with free rotation of planet carriers and ring gears to produce a contra-rotating propulsor fall short in their ability to maximize the efficiency of the system. Prior solutions impose a restraining element on only one or the other of the two output elements (planet carrier or ring gear, but not both) and offer no provision for “redistributing” this restraining energy back into the system.